1. Field of the Invention
The present invention relates to a power supply device that includes a voltage resonant circuit.
2. Description of the Related Art
FIG. 7 is a conventional power supply device that causes a backlight of a liquid crystal display device to glow. FIG. 8 is a detailed circuitry diagram of the conventional power supply device.
The conventional power supply device includes a piezoelectric transformer 10, a direct current (DC) power supply 300, a cold cathode tube 200 as a load, a current detecting circuit 11, a control Integrated Circuit (IC) 120, and a driving circuit 13. The piezoelectric transformer 10 boosts a DC voltage from the DC power supply 300 at a predetermined step-up ratio, and outputs the DC voltage to the cold cathode tube 200. The current detecting circuit 11 detects a tube current flowing through the cold cathode tube 200. The control IC 120 generates a drive control signal to control the step-up ratio of the piezoelectric transformer 10 based on the detected current. The driving circuit 13 outputs to the piezoelectric transformer 10 a driving voltage of a predetermined oscillation frequency based on the control signal.
The control IC 120 includes a brightness adjusting circuit 121 that converts the tube current detected by the current detecting circuit 11 into half-wave sinusoidal voltage as a feedback voltage to adjust brightness, a standard voltage setting unit 122 that presets a standard current, an integration circuit 123 that receives and integrates the feedback voltage based on the standard current to output an integrated value, and an operating amplifier 124 that controls and calculates the control signal such that the integrated value becomes zero. The standard voltage setting unit 122, the integration circuit 123, and the operating amplifier 124 form a voltage controlled oscillator (VCO) in the control IC 120.
A power supply control operation of the conventional piezoelectric transformer is explained next. First, an activating signal ON is input to an activating circuit 17 (see FIG. 7), and the control IC 120 is supplied with voltage Vcc, and is activated.
Due to activation of the control IC 120, the driving circuit 13 outputs the voltage Vcc from the DC power supply 300 to the piezoelectric transformer 10. Based on the predetermined step-up ratio, the piezoelectric transformer 10 supplies power to the cold cathode tube 200, thereby causing the cold cathode tube 200 to start glowing. The current detecting circuit 11 detects the tube current generated when the cold cathode tube 200 is lit. The brightness adjusting circuit 121 converts the detected current into detected voltage, and inputs the detected voltage to the integration circuit 123 with standard voltage. Based on brightness set by input through a brightness setting unit (not shown) according to preference of a user of a display device that includes the cold cathode tube 200 as a backlight, the brightness adjusting circuit 121 adjusts brightness to the brightness set by the user. For example, the brightness is set to achieve a predetermined contrast by surrounding light.
Upon receiving the detected voltage as a feedback signal, the integration circuit 123 increases output voltage and lowers oscillating frequency of the VCO. The lowered oscillating frequency output from the VCO approximates resonant frequency of the piezoelectric transformer 10. Accordingly, the step-up ratio of the piezoelectric transformer 10 increases, and the cold cathode tube 200 lights up when the voltage reaches a point where the cold cathode tube 200 starts lighting.
To adjust the brightness of the cold cathode tube 200, based on a standard voltage Vref output from a standard voltage setting unit 14a, a voltage control circuit 14 drive-controls a step-down chopper circuit 15, thereby adjusting the duty ratio of DC voltage. Consequently, the regulated DC voltage is output from the DC power source 300 to the driving circuit 13. According to the duty ratio, the driving voltage output from the driving circuit 13 is output to the piezoelectric transformer 10, and a voltage based on the predetermined step-up ratio is applied to the cold cathode tube 200. Thereby, the brightness of the cold cathode tube 200 is adjusted.
In the conventional power supply device, the step-up ratio cannot change according to a change in the input voltage of the DC power supply 300 that is an input power source, and a stable power supply cannot be ensured.
In other words, in the conventional power supply device, input to the piezoelectric transformer 10 changes due to a change in the input voltage of the DC power supply 300. For example, if the input voltage of the DC power supply 300 changes from 6 volts to 22 volts, applied voltage to the piezoelectric transformer 10 is resonate to several times the input voltage and directly applied as a resonant voltage. Due to such a change in the input voltage, for ensuring a constant output, driving frequency of the piezoelectric transformer 10 is changed according to the input voltage, and the step-up ratio of the piezoelectric transformer 10 is controlled to maintain the output constant. If the input voltage changes from 6 volts to 22 volts, the driving frequency of the piezoelectric transformer 10 must be changed by around 20 Kilo-Hertz (kHz). The driving frequency is 54 kHz for the input voltage of 6 volts, and the driving frequency is 74 kHz for the input voltage of 22 volts. According to characteristics of the piezoelectric transformer 10, efficiency of the piezoelectric transformer 10 is 92 percent at a frequency of 54 kHz which is near a resonance point, while the efficiency of the piezoelectric transformer 10 significantly reduces to 40 percent at a frequency of 74 kHz away from the resonance point. Especially, when controlling brightness of a cold cathode tube that is used as a backlight in the liquid crystal display device such as a laptop computer by using a power supply device that uses a piezoelectric transformer, a brightness difference of more than 10 cd/m2 occurs in the screen (at the center) depending on characteristics of the liquid crystal.